The present invention relates to a power conversion apparatus using a current-controlled semiconductor element as a switching element. In particular, the present invention relates to a switching-element driving device in such a power conversion apparatus. More specifically, the present invention relates to a technique for enhancing the power conversion efficiency of a power conversion apparatus using a semiconductor element as a switching element.
In view of efficient utilization of energy, a power conversion apparatus using a semiconductor switching element as a switching element has an extremely widespread availability due to its excellent characteristics in power conversion efficiency. The semiconductor switching element includes a voltage-driven type element, such as an isolated-gate bipolar transistor (IGBT), static-induction transistor and field-effect transistor (FET), and a current-driven type element, such as a bipolar-mode static-induction transistor (BSIT) and bipolar junction transistor (BJT).
The voltage driven type element may be driven directly by a voltage signal so that a driving circuit may be readily simplified and its driving frequency may also be arranged higher. In applications requiring a withstand voltage of 250V or more, several types of switching elements are selectively used depending on requirements for capacity and driving frequency. Specifically, in case of using the switching elements in a driving frequency range of several KHz to several hundred KHz, the IGBT excellent in overall balance of voltage drop in ON state and switching performance and the FEA having small current capacity but capable of high speed operation are widely employed in the power conversion apparatus.
On the other hand, since the current-driven switching type element is driven by applying current to a control terminal, a driving circuit tends to be complexified and to have a lower operation speed than that of the voltage-driven type element. However, the current-driven type switching element has an advantageous feature that the voltage drop in ON state is about one-third to one-sixth of that of the voltage-driven type element, and thereby provides a lower conduction loss. This proves that the current-driven type switching element is more suitable for providing a downsized power conversion apparatus.
While there are broadly classified two types of semiconductor switching elements available for the power conversion apparatus, as described above, it has been often the case that the voltage-driven type switching element having a low switching loss and facilitating a high frequency driving was employed in view of downsizing of components, simplification of circuits, downsizing based on high driving frequency, cost reduction and other. However, considering how to coping with social needs for achieving an enhanced efficiency and downsizing with an eye to the future, the level of voltage drop in ON state of the voltage-driven type element will be an obstacle as long as holding over the technique using the current voltage-driven switching element. For instance, observing the current situation, the voltage drop in ON state of the IGBT et al. being a mainstream voltage-driven switching element has already been improved closely up to the theoretical value. All the more because of its current high percentage of completion, it cannot be expected to reduce the conduction loss drastically.
As to switching loss, loss recovery techniques utilizing resonance phenomenon and soft switching techniques have been developed for preventing electromagnetic environment pollution and reducing power loss. In contrast, a conduction loss in the semiconductor switching element inevitably arises when a current is passed through the element and the level of the loss depends on the performance of the element. Thus, the conduction loss cannot be readily reduced only by a simple modification but a radical review of circuit topology.
Two primary losses arise in the semiconductor switching element of the power conversion apparatus; one is a switching loss arising in the course of changing the state of the semiconductor switching element from ON state to OFF state or from OFF state to ON state; and the other is a conduction loss caused by a voltage drop arising in the semiconductor switching element when this semiconductor switching element is in ON state. Thus, in order to provide a power conversion apparatus capable of meeting the need in response to the demand for further downsizing the current power conversion apparatus and enhancing its power density, it is necessary to develop a technique capable of achieving higher efficiency by comprehensively reducing both of the conduction loss caused by the voltage drop in ON state of the semiconductor switching element and the switching loss which lead to a power loss.
Heretofore, there have been very few cases reporting that the conduction loss in the semiconductor switching element was reduced by an effective improvement in circuit. Giving some examples from among such few cases, Japanese Patent Laid-Open Publication No. Hei 1-97173 discloses a technology for reducing both a switching loss and conduction loss in a PWM full-bridge power conversion apparatus, such as a PWM inverter, by applying a semiconductor switching element having a small conduction loss, such as a bipolar transistor, to an arm switched by commercial frequency, and a semiconductor switching element having a small switching loss, such as a static-induction transistor, to an arm switched by high-frequency, so as to make up a bridge circuit in the apparatus. The Journal of the Institute of Electrical Engineers of Japan, Section D, vol. 116, No. 12, 1996, pp. 1205-1210, also discloses a modification in circuit for reducing a conduction loss in a power conversion apparatus using semiconductor switching elements. However, these prior arts involve insufficient studies in terms of optimization of the conduction loss, reduction of the loss in their driving circuit, downsizing et al. For example, the aforementioned Japanese Patent Laid-Open Publication includes no specific teaching about how to drive the bipolar transistor serving as a current controlled switching element. However, when a constant current is applied to a base of the transistor as in conventional methods for driving transistors, the efficiency in low load will be particularly deteriorated due to the driving loss in no load state or low load state. In the technique described in the aforementioned Journal of the Institute of electrical Engineers of Japan, since a driving current is supplied to the transistor by a current transformer (CT), a base current is defined by the coil ratio of the CT. Thus, it is necessary for the circuit to be designed in consideration of the minimum current amplification factor of the semiconductor switching element. As a result, the semiconductor switching element will be driven to its oversaturated state during the low load state. In addition, the driving current may be effectively supplied only by relatively high driving frequency because of using the CT.
In view of the above problems, it is an object of the present invention to provide a power conversion apparatus using a semiconductor switching element and a method therefor, capable of reducing a power loss by regenerating power, and comprehensively reducing a switching loss and conduction loss arising in the switching element so as to achieve high efficiency.
It is another object of the present invention to provide a power conversion apparatus using a semiconductor switching element, capable of comprehensively reducing a switching loss and conduction loss arising in the switching element so as to achieve high efficiency.
In order to achieve the above objects, according to one aspect of the present invention, in a power conversion apparatus including a current-controlled semiconductor switching element having a collector, an emitter and a base, a switching-element driving device is provided with a current transformer having a primary winding connected in series with the switching element. A driving power supply for the switching element having rectifier means is connected to a secondary winding of the current transformer. An output of the driving power supply is supplied to the base of the switching element through a driving switch. Further, there are provided collector-emitter voltage detecting means for detecting a voltage between the collector and emitter, or a collector-emitter voltage, of the switching element, a regenerating circuit for supplying a regenerative power from the output of the driving power supply to another section having a power demand, and a collector-emitter voltage control circuit. The collector-emitter voltage control circuit controls the regenerative power to be supplied from the regenerating circuit to said another section in response to the collector-emitter voltage signal from the collector-emitter voltage detecting means so as to vary a base current to be applied to the base of the switching element and thereby control the collector-emitter voltage.
In particular type transistors, the terms xe2x80x9cdrainxe2x80x9d, xe2x80x9cgatexe2x80x9d and xe2x80x9csourcexe2x80x9d are used instead of the terms xe2x80x9ccollectorxe2x80x9d, xe2x80x9cbasexe2x80x9d and xe2x80x9cemitterxe2x80x9d. The terms xe2x80x9ccollectorxe2x80x9d, xe2x80x9cbasexe2x80x9d and xe2x80x9cemitterxe2x80x9d herein are intended to encompass the these cases; the term xe2x80x9ccollectorxe2x80x9d includes the drain, the term xe2x80x9cbasexe2x80x9d including the gate, and the term xe2x80x9cemitterxe2x80x9d including the source.
In the preferred embodiment of the present invention, there are provided a driving power supply for supplying a current for driving the switching element, and a reverse bias circuit for applying a reverse bias to the base of the switching element. The collector-emitter voltage control circuit includes on-driving switching means adapted to connect the driving power supply to the base of the switching element, off-driving switching means adapted to connect the reverse bias power supply to the base of the switching element, and control means for controlling the regenerative power based on the collector-emitter voltage signal received from the collector-emitter voltage detecting means so as to control the base current to be supplied to the base of the switching element and thereby control the collector-emitter voltage. The on-driving switching means is conducted and the off-driving switching means is shut off when the switching element is turned on, while the off-driving switching means is conducted and the on-driving switching means is shut off when the switching element is turned off, so as to allow the switching element to be rapidly turned off by the reverse bias from the reverse bias power supply.
In the present invention, an activating power may be supplied from said section adapted to be supplied with the regenerative power to the driving power supply during an activation period of the driving power supply. The collector-emitter voltage control circuit may include switching means for controlling the regenerative power by the switching operation of the switching means so as to vary the base current to be applied to the base of the switching element, and rectifier means provided at an output section of the switching means. In this case, the rectifier means may include a rectifier element and an auxiliary rectifier element having a lower conduction resistance than that of the switching means.
In another embodiment of the present invention, the switching-element driving device may include temperature detecting means for detecting a temperature of the switching element, and a current control section adapted to store an optimum data of the collector-emitter voltage corresponding to plural different temperature values of the switching element, and define an optimum value of the collector-emitter voltage based on a temperature signal from the temperature detecting means.
According to another aspect of the present invention, there is provided collector-emitter voltage detecting means for detecting a voltage between a collector and an emitter of a semiconductor switching element in a power conversion apparatus. The collector-emitter voltage detecting means is adapted to control a base current to be supplied to the switching element based on the detected collector-emitter voltage so as to provide an optimum driving in consideration of factors including dispersion in a specific current amplification factor (hfe) of the switching element, variance in the hfe caused by temperature, and variance in the hfe to a current flowing through the switching element, for reducing a sum of conduction loss and driving power of the switching element.
More specifically, according to the aforementioned aspect of the present invention, there is provided a switching-element driving device in a power conversion apparatus including a current-controlled semiconductor switching element having a collector, an emitter and a base. This switching-element driving device according to the present invention includes an output main line connected to the base of the switching element, an output return line connected to the emitter of the switching element, and a collector-emitter voltage control means. The collector-emitter voltage control means includes the collector-emitter voltage detecting means for detecting the voltage between the collector and the emitter of the semiconductor switching element. The collector-emitter voltage control means is adapted to control a base current of the semiconductor switching element supplied to-the output main line based on the detected collector-emitter voltage so as to control the collector-emitter voltage to reduce the sum of conduction loss and driving power in the semiconductor switching element.
In another embodiment of the present invention, the switching-element driving device may include a driving power supply for supplying a current for driving the switching element, and a reverse bias means for applying a reverse bias to the base of the switching element. The collector-emitter voltage control means includes on-driving switching device adapted to connect the driving power supply to the base of the switching element, off-driving switching device adapted to connect the reverse bias power supply to the base of the switching element, and control means for controlling a base current supplied to the base of the switching element based on a collector-emitter voltage signal received from the collector-emitter voltage detecting means so as to control the collector-emitter voltage. In this case, the on-driving switching means is conducted and the off-driving switching means is shut off when the switching element is turned on, while the off-driving switching means is conducted and the on-driving switching means is shut off when the switching element is turned off, so as to allow the switching element to be rapidly turned off by the reverse bias from the reverse bias power supply.
In another embodiment according to the present invention, the collector-emitter voltage control means may include a current control section for storing an optimum data of the collector-emitter voltage of the switching element, and supplying to the base of the switching element the base current controlled based on the stored data and a collector-emitter voltage signal from the collector-emitter voltage detecting means. In this case, the switching-element driving device may include temperature detecting means for detecting a temperature of the switching element, and the current control section may be adapted to store the optimum data of the collector-emitter voltage corresponding to plural different temperature values of the switching element, so as to define an optimum value of the collector-emitter voltage based on a temperature signal from the temperature detecting means. Further, the switching-element driving device may be formed in a luminescence structure having current detecting means for detecting a collector current flowing through the collector of the switching element. The collector-emitter voltage control means may be adapted to store the optimum data of the collector-emitter voltage corresponding to plural different current value of the switching element so as to define an optimum collector-emitter voltage value based on a collector current signal from the current detecting means. Furthermore, the collector-emitter voltage control means may include base current control switching means for variably controlling the base current of the switching element by the switching operation of the base current control switching means, and rectifier means provided at an output section of the base current control switching means, and the rectifier means may be formed in a synchronous rectifier having a rectifier element and an auxiliary rectifier element having a lower conduction resistance than that of the base current control switching element.
The present invention also provides a method of driving a switching element. This method comprises the steps of obtaining a power from a secondary winding of a current transformer having a primary winding connected in series with the switching element, supplying a part of the obtained power to another section having a power demand as a regenerative power, detecting a collector-emitter voltage of the switching element as obtaining a driving current to be applied to a base of the switching element from the remaining power so as to drive the switching element, and controlling the regenerative power based on the detected collector-emitter voltage value so as to vary the driving current to be applied to the base of the switching element, and thereby control the collector-emitter voltage.